Children's mathematical knowledge at school entry varies considerably and predicts long-term achievement outcomes. Differences in children's exposure to math and number talk at home may help to explain variations in school-entry math ability. However, nearly all research on exposure to math and number talk has been conducted with parents and preschool-aged children. This longitudinal study examined to what extent parents talk about math with their 10-, 14- and 18-month infants during play. Additional analyses examined factors to explain variation in parent math talk at the family level (socio-economic status), parent level (beliefs about children's mathematical development) and child level (gender). Transcripts of dyadic play sessions were coded for references to numbers and math concepts (e.g., first, many, more). It was found that parents do talk about numbers and math concepts with infants, albeit rarely, and such language increases in frequency and complexity over time. Parents use more statements than questions about math, and most often reference small-number terms (1-3). Neither socio-economic status nor beliefs were predictive of parental math talk. However, parents used significantly more math references over time with boys than girls. The results are discussed in terms of the importance of social interactions during infancy in fostering children's mathematical knowledge.